Tuned filters that have broad tuning range and high signal selectivity are used in spectrum analyzers and other measurement systems. The passband center frequency of a tuned filter is adjustable via control signals that are applied to the filter. In a spectrum analyzer, measurement accuracy and dynamic range depend upon how accurately the control signals can tune or adjust the center frequency of the filter to track other signals such as the local oscillator signal within the spectrum analyzer. Typically, the tuning characteristic of a tuned filter, such as a yttrium-iron-garnet (YIG) tuned filter or YTF, is obtained and stored in memory during a baseline characterization of the spectrum analyzer. The stored tuning characteristic determines the control signal level needed to achieve a specified passband center frequency. However, temperature variations and changes in operating parameters of the spectrum analyzer alter the tuning characteristic of a YTF so that accurate adjustment of the center frequency of the YTF is not achieved using the tuning characteristic stored in memory.
Various calibration schemes have been used to compensate for changes in the tuning characteristic of the YTF relative to the tuning characteristic stored in memory. In a first calibration scheme microwave signals supplied by an external signal source are injected into the YTF at predefined frequencies within the tuning range of the YTF. Based on the transmission properties of the YTF at the predefined frequencies, a correspondence between center frequencies and control signal levels is obtained. This correspondence is used to determine the control signal levels needed to accurately adjust the center frequency of the YTF so that tracking between the center frequency and the local oscillator signal in the spectrum analyzer is provided. This calibration scheme relies on an external signal source and requires physically connecting the source to the YTF each time the calibration is performed, which is time-consuming. In a second calibration scheme microwave signals supplied by an internal signal source, such as a comb generator, are injected into the YTF at predefined frequencies within the tuning range of the YTF. In this scheme, the transmission properties of the YTF are also used to obtain a correspondence between the center frequencies of the YTF and control signal levels. However, the internal signal source increases the manufacturing cost of the spectrum analyzer. This calibration scheme is also time-consuming because it involves physically rerouting signal paths within the spectrum analyzer to apply the microwave signals from the internal signal source to the YTF.
Typically, temperature and operating parameters of a spectrum analyzer vary as measurements are performed. Therefore, the tuning characteristic stored in memory during the baseline characterization requires correction or calibration sufficiently often to assure that the center frequency of the YTF can be accurately adjusted so that accurate tracking to the local oscillator signal is provided. Accordingly, there is a need for a filter calibration system that can calibrate the tuning characteristics of a tuned filter, such as a YTF, in a time-efficient manner without increasing the manufacturing cost of the spectrum analyzer.